1. Field of the Invention
The invention relates to a device for diagnosing catalyst deterioration on the basis of the decrease in oxygen storage capacity.
2. Description of Related Art
Some of the catalysts disposed in exhaust passages of internal combustion engines installed on vehicles have oxygen storage capability. The properties of such catalyst are such that when the air/fuel ratio of exhaust gas flowing to the catalyst is leaner than the stoichiometric air/fuel ratio, excess oxygen contained in the exhaust gas is adsorbed and retained, and where the air/fuel ratio of exhaust gas flowing to the catalyst is richer than the stoichiometric air/fuel ratio, the adsorbed and retained oxygen is released. In such catalyst, even if the actual air/fuel ratio somewhat deviates from the stoichiometric air/fuel ratio according to the operation state, this deviation of air/fuel ration can be absorbed by the oxygen storage/release action of the catalyst and the exhaust gas purification efficiency can be maintained.
Deterioration of such catalysts can be diagnosed on the basis of the decrease in oxygen storage capacity, that is, on the basis of the decrease in the maximum limit amount of oxygen that can be stored. The oxygen storage capacity of a catalyst can be measured by a Cmax method. The Cmax method is performed by conducting active air/fuel ratio control in which the air/fuel ratio of the exhaust gas flowing to the catalyst is forcibly switched to the rich side and lean side and Cmax, which is an index of catalyst oxygen storage capacity, is determined as an average value of two integral values, namely, the integral value OSAFALL of oxygen release amount during rich control and the integral value OSARISE of oxygen storage amount during lean control.
A device described in Japanese Patent Application Publication No. 2010-101211 (JP-A-2010-101211) is available as a device for diagnosing catalyst deterioration. In the device described in JP-A-2010-101211, the air/fuel ratio difference among cylinders is corrected when exhaust gas recirculation (EGR) is not implemented and then the introduction of EGR is restarted and the air/fuel ratio difference among cylinders is corrected again in this state. Catalyst deterioration is then diagnosed upon correcting the air/fuel ratio difference among cylinders after the EGR has been restarted, thereby making it possible to perform appropriate diagnostic of catalyst deterioration when the EGR is implemented.
Thus, in the device described in JP-A-2010-101211, catalyst deterioration is diagnosed when the EGR is implemented. However, when an internal combustion engine is operated, the EGR is sometimes introduced, but sometimes the EGR is not introduced. In particular, in cold climates, the operation period can mostly correspond to a semi-warmed-up state. Meanwhile, the EGR is generally implemented only when warm-up is completed. Therefore, in cold climates, the period in which the operation is performed in a state without EGR introduction is extended. Further, in a hybrid car that is provided with two drive sources, namely, an internal combustion engine and an electric motor, when the car is driven by the motor or the car is stopped, the internal combustion engine is stopped. Therefore, in winter, engine operation is mostly performed in a state in which the warm-up has not yet been completed and the period in which the operation is performed in a state without EGR introduction is likewise extended. For this reason, in order to ensure ample opportunities for diagnostic, it is desirable that catalyst deterioration be diagnosed both when the EGR is implemented and when it is not implemented.
However, in internal combustion engines installed on hybrid cars, EGR introduction is performed at a high EGR ratio equal to or higher than 15%. The results of the examination conducted by the inventors have clearly indicated that in an internal combustion engine with such large-volume EGR the catalyst state differs significantly between a period in which the EGR is implemented and a period in which it is not implemented.
FIG. 10 shows the relationships between the Cmax and intake air amount of the internal combustion engine obtained when the EGR is implemented and when it is not implemented. The relationships have been measured by using the catalyst with the same degree of deterioration. As clearly follows from the figure, when the EGR is not implemented, the Cmax, that is, the oxygen storage capacity of the catalyst, is clearly higher than that in the implementation period. In this connection, when the EGR is implemented (EGR is present), the exhaust gas temperature is lower and the temperature distribution of the catalyst is accordingly somewhat lower, as shown in FIG. 11, than when the EGR is not implemented (EGR is absent). However, this difference in oxygen storage capacity cannot be explained merely by such a difference in temperature distribution of the catalyst.
The oxygen storage capability (capacity) of the catalyst thus differs significantly between a period in which EGR is implemented and a period in which it is not implemented. For this reason, adequate diagnostic of deterioration cannot be performed when the diagnostic is conducted in the same mode when the EGR is implemented and when it is not implemented.